Method of and a device for providing upon cathode carriers emission layers and a cathode prepared thereby for the operation thereof



Feb. 28, 1967 R. LAUTER 3,305,796

METHOD OF AND A DEVICE FOR PROVIDING UPON CATHODE CARRIERS EMISSIONLAYERS AND A CATHODE PREPARED THEREBY FOR THE OPERATION THEREOF FiledMay 15, 1962 Fig.1

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United States Patent ration of Germany Filed May 15, 1962, Ser. No.194,908 Claims priority, applicsati gr Germany, May 16, 1961,

6 Claims. or. 156155) The invention disclosed herein is concerned with amethod of and a device for providing thin emission coatings or layersupon carriers for directly or indirectly heated cathodes of electricaldischarge vessels, comprising placmg upon the smooth surface of apliable supporting film a layer of an emission paste intermixed withbinder and softener agents, and thereupon bonding such supporting filmwith its emission layer to a cathode carrier, and with a cathode bodyprepared thereby for operation thereof in a discharge vessel.

It is in addition to orthodox methods of producing emission layers,which have considerable drawbacks, especially in the processing withautomatically operating machinery, known to produce emission layers bymaking a paste layer which is bonded to a cathode carrier. For example,it is known to produce a paste layer by spraying or pouring a paste uponthe smooth surface of a support, for example, a glass support, whereuponsuch layer is after drying thereof peeled from the support and bonded toa cathode carrier such as a tantalum wire spiral. However, it is inpracticing such method, owing to the rigidity of the relatively thickglass support very difiicult to peel or to lift therefrom a compactpaste layer and to place such layer upon the carrier surface, withoutdamaging the layer, even when a considerable amount of binder materialhad been added to the paste.

Another known method avoids part of these drawbacks by putting the pastefilm upon a pliable support from which the layer can be more readilyremoved.

The bonding of the paste to a cathode carrier requires in the knownmethod that it be removed from the support prior to the bonding orincident thereto, and it is therefore necessary that the paste layer hasa particularly good cohesion, which is obtained by the use of increasedamounts of customary binding agents. However, it is known that seriousdifficulties appear in the operation of cathodes having emission layerscontaining an appreciable amount of binder material.

The object of the invention is to propose a method of producing veryuniform and dense emission layers with smooth surfaces, which areadapted for mass production, and in which the above described drawbacksare to a large extent avoided.

This object is in accordance with the invention realized by placing upona very thin supporting film a paste layer consisting substantially ofalkaline earth carbonate and thereupon bonding such supporting film withits paste layer to a cathode carrier or cathode shell.

The support used for this purpose is a film of synthetic material of acomposition, such that it decomposes in the discharge vessel incident tothe decomposing of the paste layer taking place in the cathodeactivation, without leaving any disturbing residue or liberatingaggressive gases. A solution of dinitrocellulose, such for example, as asolution of collodion cotton, which is commercially available under thedesignation E1440, is advantageously employed for making the filmoperating as a support for the emission layer. The solvent for theinitial solution for making the film must be selected with considerationof the fact ice that the emission layer is placed thereon throughout inthe form of an emulsion, namely, an emission paste which contains anapproximately identical or a similar binder, thus containingcorresponding solvents, so that the produced film support is in no waycompletely dissolved again incident to receiving the emission layer, bythe solvent contained in the latter, but is merely partially partiallyslightly softened, thereby securing a good adhesion of the paste of theemission layer on the support. Based upon this recognition and upontesting various solvents and compositions, the following composition ofa solution was found to be particularly suitable, namely:

Dinitrocellulose (e.g., collodion cotton El440) g 2O Butoxyl ml 200Butylacetate ml 400 Butylglycol ml 200 Polyacrylacidester 40% dissolvedin xylol ml 200 A film made with the aid of this solution has a considerably greater strength than a film permeated with filling substrances,such, for example, as alkaline earth carbonates.

The compound foil comprising the paste layer with the supporting filmtherefor, is in particularly advantageous manner placed upon the cathodecarrier by bonding the paste layer directly to the cathode carrier, withthe supporting film facing to the outside. The cathode carrier canthereby be Wetted with the solvent so as to obtain a good bonding effectand therewith good adhesion of the paste layer on the cathode carrier.

The procedure explained above results, as compared with other methods,in very important advantages. A cathode produced in such manner is apriori provided with a protective film which prevents damage to thesensitive emission paste layer in the course of assembly operations,which is particularly true when considering the very small grid-cathodespacings customarily provided in present day tube structures. Moreover,as contrasted with previously known methods, the emission paste layer isupon placement thereof on a cathode carrier, in nomanner subjected tomechanical stresses. Accordingly, there is no need for increasing thebinder component so as to obtain good cohesion. It is indeed possible inusing the described method, to reduce the binder content used, forexample, in the spraying procedure, to an amount lying below that usedin connection with prior methods. As a consequence, the cathodesproduced in accordance with the present invention, as contrasted withpreviously known cathodes, show no tendency of blackening as a result ofdecomposition of the corresponding carbonate layer in connection withthe activation thereof in the discharge vessel.

The blackening of cathodes occurs particularly when the binder, which assuch burns completely to form gaseous compounds, namely, oxygen of thecarbon, hydrogen and nitrogen, is in the manner of pigments permeatedwith alkaline earth carbonate (or rather the latter permeated bythebinder) causing-incomplete decomposition and thereby producing amongothers, free carbon. The produced free carbon has the property ofstrongly radiating heat, whereby the cathode temperature may be reducedto a value below the optimum operating value. In addition, the freecarbon can react to form carbides with the free barium in the layer,which is responsible for the activity of the cathode, thereby harmingthe emission. The carbides as such perform high efilux work (pooremission) and can moreover form hydrocarbon substances with Water vaporin the residual gas in the tube, such hydrocarbon substances causing inthe manner of a cycling process a continuous consumption of theactivating barium of the emission layer (continuous poisoning of thecathode).

Further details of the invention will now be described with reference tothe accompanying drawing.

FIG. 1 shows at the right hand portion thereof part of a supporting filmwith an emission paste layer provided thereon, and at the left portionthe manner in which such a compound foil is bonded to a cathode, atubular cathode being indicated to give an example;

FIG. 2 illustrates the manner of producing the support ing film; and

FIG. 3 indicates the manner of producing the emission paste layer.

The strip-shaped so-called compound foil, a portion of which is shown atthe right hand part in FIG. 1, cornprises the supporting film 3 and theemission paste layer 2. Both, the film and the layer as a unit, aresubdivided into sections corresponding in width, for example, in thecase of tubular cathodes, to the length of the latter. The left part ofthe figure shows the manner in which a section of the compound foil isby a rolling operation bonded to a cathode carrier, such, that theemission paste layer 2 is in direct engagement with the metallic carrier1 while the supporting film 3 faces outwardly. The paste layer isthereby in no way subjected to mechanical stresses but is in fact in thedirection of the cathode carrier body continuously even though slightlydensified.

It is of course understood that, if needed, for example, in connectionwith irregularly shaped cathodes, the two layers can be arranged inreversed sequence with respect to the cathode body.

The emission paste layer can be produced either in accordance with priormethods or, in particularly advantageous manner, by spreading anddrawing the layer over a support. The supporting film may be producedsimilarly.

FIG. 2 shows on a greatly enlarged scale the manner of making thesupporting film. The lower end of a suitable supply receptacle, whichcontains an appropriate solution or suspension, forms or is connectedwith a capillary nozzle 4 having a slot-like opening through which thesolution is discharged. The angularly shaped discharge end of thisnozzle, With the fluid discharging therefrom, is drawn over a smoothsupport in a direction extending in parallel therewith, with the edge ofits longer side in engagement with the support, the edge of its shorterside thereby defining with respect to the support a very narrow spacethrough which the fluid issues, thus leaving on the support a very thinfilm 3 of synthetic material in the form of a long strip of desiredwidth. The amount of fluid which is being discharged per time unit issubstantially determined by the size of the capillary discharge openingand its spacing from the support. The speed of motion of the nozzlealong the support can be varied depending upon the viscosity of thefluid so as to obtain in any case a foil of very exact and particularlyuniform thickness. It is of course understood that it is possible tooperate with a stationary nozzle and to move the support with respectthereto.

The important individual values must be empirically established in viewof the considerable shrinkage occurring during the drying of thesupporting film made in the above described manner. A practical valuefor the width of the nozzle is, for example, 0.3 millimeter which isrelatively great compared with the thickness of the film which mayamount to 8 up to 10 microns.

After the solvent has sufiiciently evaporated, the film can be severedfrom its support, for example, by immersion in water, and can be driedat approximately room temperature by air blown against it.

However, the operation will generally be such that the film, while beingon its support, is provided with the emission paste layer and is, afterappropriate drying in air, removed from the support as a unit togetherwith the paste layer, at a temperature of about 100 C.

It is in addition to the above explained properties necessary that thesupporting film withstand the drying temperature for the emission pastelayer without undergoing any changes.

As shown in FIG. 3, the emission paste layer may be advantageouslyproduced in a similar manner as the supporting film, utilizing thelatter as a support therefor. The emission paste is contained in asuitable container provided with a nozzle 4 having a discharge openingwhich is narrower than that of the corresponding nozzle for the filmfluid, as shown in FIG. 2, such opening being, for example, of a sizefrom 0.1 to 0.13 millimeter. The nozzle is moved with respect to thepreviously prepared supporting film 3, leaving thereon an emission pastelayer 2. The solvent contained in the paste fluid efifects slightpartial dissolution or softening of the supporting film, therebyproviding an increased bonding action and producing the desired compoundfoil.

Both, the emission layer and the supporting film can be produced ascontinuous strips with the aid of appropriate automatic machinery, forexample, with the aid of a foil drawing machine.

The emission paste layers produced according to the invention have adensity of about 1.3, that is, a relatively high density, as comparedWith layers made, for example, by spraying, in accordance with a priormethod. A subsequent densifying, for example, by pressing, as previouslyproposed, is therefore unnecessary in connection with compound foils oremission layers made as described herein.

The important advantages obtained by emission layers made according tothe invention, as compared with emission layers produced with the aid ofprior methods, reside primarily in that the compound foils, consistingof a supporting film and an emission paste layer, have greater strengthand good flexibility, and that they are, as compared with the knownseparable paste films, practically not at all mechanically stressed.Since the amount of binder material in the emission paste layer can beheld very small, there is no danger of formation of black car'- bonatelayers (residual carbon). The construction of the compound foil providesfor the emission paste layer, after the latter is joined with thecathode body, a protective layer (supporting film) which prevents damagethereto that may be caused incident to the assembly operation in view ofthe small grid-cathode spacing to be realized in: present day tubes. Thesurface of the emission layer (paste layer) made according to theinvention, which ad joins the supporting film, is particularly smooth.HoW-' ever, this smooth surface of the emission layer is afterdecomposition of the supporting film exposed, that is, it will face thegrid cooperating with the cathode. Since the supporting film is in acathode prepared for operation as described herein, disposed on theoutside, that is, on the side facing away from the cathode body, thereis assurance that it will completely decompose without leavingdisturbing residues. The invention is applicable for pro ducing emissionlayers in connection with the practically all forms of cathodes andlikewise in connection with mass production, providing nevertheless veryuniform and pri marily dense and very smooth layers.

The paste for forming the emission layer may be ground and emulsified toprovide a particle size which is smaller than about 6 microns.

Changes may be made within the scope and spirit of the appended claimswhich define what is believed to be new and desired to have protected byLetters Patent.

I claim:

1. A method of forming thin emission layers upon carriers for directlyor indirectly heated cathodes of elec trical discharge vessels,comprising placing upon the smooth surface of a very thin pliablesupport film, substantially made of dinitrocellulose of a thickness from8 10 microns, an emission layer in the form of a paste consistingsubstantially of an alkaline earth carbonate intermixed with a binder ofdinitrocellulose and softener agents, with the binder facilitatingadhesion of said paste to said film to produce a compound foil formed ofsaid film and layer, bonding said compound foil to said cathode memberwith the emission layer thereof in engagement with such member, and uponoperation of the cathode, incident to decomposing action of the emissionlayer, effecting decomposition of said film Without leaving anydisturbing residue or liberating aggressive gas.

2. A method according to claim 1 comprising the step of wetting saidcathode member with a solvent prior to bonding said foil thereto.

3. A method according to claim 1, wherein said compound foil is formedupon a support from which it is after drying thereof removed at atemperature of about 100 C.

4. A method according to claim 1, wherein said paste is a suspension ofalkaline earth carbonate containing a binder of dinitrocellulose in theamount of about 2 to 2.5 percent by weight, said paste being applieddirectly to said film.

5. A method according to claim 1, comprising grinding and emulsifyingsaid paste so as to make the particle size thereof smaller than 6microns.

6. A method according to claim 1, wherein said film is made from acomposition containing 24 g. dinitrocellulose, 200 ml. butoxyl, 400 ml.butylacetate, 200 ml. butylglycol, and 200 ml. polyacrylacidesterdissolved in 40% xylol.

References Cited by the Examiner OTHER REFERENCES Ser. No. 254,159,Lierg (A.P.C.), published Apr. 27, 1943, now abandoned.

EARL M. BERGERT, Primary Examiner.

L. T. PIRKEY, I. P. MELOCHE, Assistant Examiners.

1. A METHOD OF FORMING THIN EMISSION LAYERS UPON CARRIERS FOR DIRECTLYOR INDIRECTLY HEATED CATHODES OF ELECTRICAL DISCHARGE VESSELS,COMPRISING PLACING UPON THE SMOOTH SURFACE OF A VERY THIN PLIABLESUPPORT FILM, SBUSTANTIALLY MADE OF DINITROCELLULOSE OF A THICKNESS FROM810 MICRONS, AN EMISSION LAYER IN THE FORM OF A PASTE CONSISTINGSUBSTANTIALLY OF AN ALKALINE EARTH CARBONATE INTERMIXED WITH A BINDER OFDINITROCELLULOSE AND SOFTENER AGENTS, WITH THE BINDER FACILITATINGADHESION OF SAID PASTE TO SAID FILM TO PRODUCE A COMPOUND FOIL FORMED OFSAID FILM AND LAYER, BONDING SAID COMPOUND FOIL TO SAID CATHODE MEMBERWITH THE EMISSION LAYER THEREOF IN ENGAGEMENT WITH SUCH MEMBER, AND UPONOPERATION OF THE CATHODE, INCIDENT TO DECOMPOSING ACTION OF THE EMISSIONLAYER, EFFECTING DECOMPOSITION OF SAID FILM WITHOUT LEAVING ANYDISTURBING RESIDUE OR LIBERATING AGGRESSIVE GAS.